The White House signed an order letting troops at the border use force — and it may run foul of the law

A US Marine secures concertina wire at the Otay Mesa Port of Entry on California-Mexico border, November 18, 2018.
U.S. Marine Corps photo by Sgt. Asia J. Sorenson

The White House late Tuesday signed a memo allowing troops stationed at the border to engage in some law enforcement roles and use lethal force, if necessary — a move that legal experts have cautioned may run afoul of the Posse Comitatus Act.

The new "cabinet order" was signed by White House Chief of Staff John Kelly, not President Donald Trump. It allows "Department of Defense military personnel" to "perform those military protective activities that the Secretary of Defense determines are reasonably necessary" to protect border agents, including "a show or use of force (including lethal force, where necessary), crowd control, temporary detention. and cursory search."

There are approximately 5,900 active-duty troops and 2,100 National Guard forces deployed to the US-Mexico border.

Some of those activities, including crowd control and detention, may run into potential conflict with the 1898 Posse Comitatus Act. If crossed, the erosion of the act's limitations could represent a fundamental shift in the way the US military is used, legal experts said.

A US Marine stands guard behind barbed wire along the US-Mexico border in preparation for the migrant caravan. Jorge Duenes/Reuters

The Congressional Research Service, the non-partisan research agency for Congress, has found that "case law indicates that 'execution of the law' in violation of the Posse Comitatus Act occurs (a) when the Armed Forces perform tasks assigned to an organ of civil government, or (b) when the Armed Forces perform tasks assigned to them solely for purposes of civilian government."

However, the law also allows the president "to use military force to suppress insurrection or to enforce federal authority," CRS has found.

Military forces always have the inherent right to self-defense, but defense of the border agents on US soil is new. In addition, troops have been given additional authorities in previous years to assist border agents with drug interdictions, but the widespread authorization of use of force for thousands of active-duty troops is unique to this deployment.

Each domestic deployment of troops to any of the 50 states or US territories is governed by Chairman of the Joint Chiefs of Staff Instruction 3121.01B, "Standing Rules of Engagement, Standing Rules for the Use of Force by US Forces." Two annexes, L and N, are specific to Defense Department missions in support of civilian authorities.

US soldiers string razor wire near the port of entry at the US-Mexico border in Donna, Texas, on November 4, 2018.John Moore/Getty Images

However, each mission is unique, and the standing rules for the use of force can be adjusted except for the limitation against active-duty US forces conducting law enforcement on US soil, which is enshrined in the 1898 act.

Posse Comitatus is "always looming in the background. You never invoke it as such because it is such a background principle," said William Banks, author of "Soldiers on the Home Front: The Domestic Role of the American Military" and the former director of the Institute for National Security and Counter-terrorism at Syracuse University's College of Law.

Defense officials said the language in the directive was carefully crafted to avoid running up against the bedrock legal limitations set in Posse Comitatus.

The law was originally intended to protect the states from being controlled by federal troops. It has evolved into a singly defining, almost church-versus-state-type wall forbidding active-duty forces under the control of the president from conducting any types of crowd control or law-enforcement domestically, essentially ensuring that the US military is not used to control or defeat American citizens on US soil.

US soldiers work alongside US Customs and Border Protection at the Hidalgo, Texas, on border with Mexico, November 2, 2018. U.S. Air Force photo by SrA Alexandra Minor

Kelly said in the signed directive that the additional authorities were necessary because "credible evidence and intelligence" have indicated that the thousands of migrants who have now made their way to the US checkpoint near Tijuana, Mexico, "may prompt incidents of violence and disorder" that could threaten border officials.

But the White House still may find itself in a legal challenge if the authorities in the memo are determined to be counter to the law, Banks said. "Even [an executive order] couldn't overcome Posse Comitatus," Banks said.

For months, Trump has looked to the military to seal off the US-Mexico border because he has not been able to persuade Congress to fully fund a border wall.

Instead, Trump has sought to make the military's border presence more aggressive and suggested that he might send as many as 15,000 US troops to the border.

Last month, just before the mid-term elections, Trump told troops they could consider migrants throwing rocks at them the use-of-force equivalent of migrants threatening them with firearms. Under current military rules of engagement, that would authorize a return use of deadly force.

Trump later backtracked on the rifles comment, saying he never directed troops to shoot migrants and that Customs and Border Protection would take the lead in any of those scenarios.